Wave translation system



' June 4, 1935. H. s. BLACK 2,003,282

WAVE TRANSLATION SYSTEM Original Filed Aug. 8, 1923 3 Sheets-Sheet' l WATTS Qur/:0r

V I V3 R E KRO o f aV 4) H F/G'BE HA l o -@p l 5 R01 n l Z* I 3 m? |||||||||k| l mgmt Lb? KR T I #VVE/vm@ HAROLD 5. BLACK Wim/@MQW June 4, 1935. H. s. BLACK WAVE TRANSLATION SYSTEM Original Filed Aug. 8, 192B 3 Sheets-Sheet 2 wwf/vm@ HARD/ D .SBLACK @y yn/VW Arron/Vey June 4, 1935. ||.`s. BLACK WAVE TRANSLATION SYSTE original Filed Aug. 8, 192s s sheets-'smet s #s uw w/TH Fefaeac/r cA//v w/THour #Espana 80 C 40 l l0 20 30 40 50 60 70 8O E 7o. M// .AMP$. OUTPUT q lo- O l0 2O 30 40 50 60 70 8O W M/L. AMPS. FUNDAMENTAL OUTPUT INTO 600 /NVENTOR i H. 5. BLACK HVJ A TTORNEV Patented June` 4, 1935 Haroldv S. Black, New York, N. Telephone Laboratories,

Y., assignor to Bell Incorporated, New

York, N. Y., a corporation of New York Original application August 8, 1928, Serial No.

K 298,155. Divided and ber 3, 1929, Serial No. 21, 1929 17 Claims.

This is a division of my prior application Serial No. 298,155, filed August 8, 1,928 for'Wave translation systems.

This invention relates to wave translation systems, as for example systems for amplifying electrical variations with the aid of electric space discharge devices.

It is common experience that increase,V of the power output of vacuum tubes or electric space discharge devices tends to increase distortion of signaling or other waves transmitted by the devices, and tends to lower the gain of the circuits of the devices.

At high values of output power, gain may become so low that further increase of input amplitude or decrease of output impedance produces no further increase of power output.

However, in many applications of vacuum tubes, long before the power output of the tube reaches the maximum valuev that the'tubeis capable of delivering or seriously lowers the gain of th tube circuit, distortion becomes so great' as to render further increase of output power inadvisable. Representative instances in the field of application of amplifiers, for example, are voice frequency telephone repeaters, carrier frequency amplifiers common to a plurality of signaling channels in carrier wave multiplex signaling systems, amplifiers for public address systems, amplifiers for reproducing music from records, power amplifiers used as transmitting or sending amplifiers in radio or wire transmission systems, amplifiers for operating loud speaking receivers for radio sets or the like, and sending amplifiers for high quality broadcastingtransmitters.

In each such instance, the higher the quality of' transmission required, or in other words, the less the distortion permissible, the sooner is the maximum permissible output power reached, that is, the lower is the ratio of maximum permissible output power to the maximum output power that the tube or tubes of the amplifier stage is c apable of delivering. Therefore, the-higher the transmission quality demanded, the more inefficiently has the power capacity of the tube or tubes been used, and consequently when both -high transmission quality and high output of power are required in Vsystems of the types com'- monly employed, the power capacity of the tubes has to be especially great because only a small fraction of the total power that each tube is capable of delivering can be utilized. The present tendency in practice is toward higher and higher transmission quality, and also the number l this application Decem- 411,224. In Canada May of applications and installations requiring high output power is rapidly increasing, and moreover, the tendency is toward higher quality even where enormous power is demanded and interference or disturbances tending to introduce noise in the waves transmitted is great, as in trans-Atlantic radio telephony.

In general, the design limitations for vacuum tubes of the types commonly employed are such that the highest output power capacity of the tube is obtained only at a sacrifice of maximum gar obtainable, or of operating efliciency, or of bo In the case of a carrier frequency amplifier common to a plurality of signaling channels in a carrier wave multiplex signaling system, the tendency of the amplifier to modulate a wave of one frequency by a wave of another frequency, increases greatly with increase in the output power, and there results therefore a very large increase in crosstalk and interference between the various channels,`this increase being more serious the larger the number of channels.

Therefore, a major problem in devising vacuum tube systems, as for example vacuum tube amplifier systems, is the securing of high output of power without attendant disadvantages, as for example without increase of first cost or decrease of operating efficiency of the systems, and especially in the case of vacuum tube amplifiers and repeaters, without sacrifice of quality of signal reproduction.

Representative objects of the inventionvare (1) economically to increase the load carrying capacity of wave translation systems, as for example, systems for amplifying electrical variations with the aid of electric space discharge devices, (2) to control modulation in such systems, (3) to amplify waves without modulation or other distortion, (4) to facilitate the handling of large loads by electric space discharge devices, and 5) to stabilize the functioning of systems comprising electric space discharge tubes, as for example to prevent variations of tubes or power from affecting gain.

In one specific aspect the invention is a signal wave amplifying systeml of a type claimed in my copending application Serial No. 606,871, filed April 22, 1932, for Wave translation systems. In systems 'of such type, fundamental or applied components and distortion components produced in an amplifying device, as for example a vacuum tube amplifier'circuit, are so fed back as to reduce the magnitude of distortion components of both odd and even orders in the output circuit of the device for a given power output of fundamental and stabilize the gain and increase the load carrying capacity' of the system.

In its above mentioned specific aspect the 5 present invention is also a system of a type claimed in my copending application Serial No. 411,223, flledDecember 23, 1929, for Wave translation systems. In a system of that type comprising an amplifying device, in obtaining reduction of distortion, increase of gain stability and high load capacity, undesired reduction in the gain of the system is avoided by isolating the distortion components from the fundamental components and feeding back only the distortion components when the gain of the device has no tendency to depart from a normal or prescribed value, and, when such tendency exists, so controlling feedback of fundamental components as to cause reduction or prevention of departure of the gain f the system from its normal or prescribed value.

In its above mentioned specific aspect the present invention is asystem of such type in which the isolated distortion components `are so fed back that they are treated by the system the same as fundamental components applied to the system and consequently their feedback does not cause alteration of the value of the isolated distortion components. When they have been fed back they proceed through the system to the place of origin of the distortion components in the. system and arrive at that place with their amplitude and phase substantially equal and opposite, respectively, to the original amplitude and phase of theS distortion components, and

therefore cancel or neutralize the original distortion components, with the result that the distortion is substantially zero in the output of the system.

- Other objects and aspects of the invention will be ,apparent from the following description and claims. f

Figs. 1 and 2 ofthe drawings are circuit diagrams for facilitating explanation of the inven- 5 tion; Fig. 3 shows a feed-back amplifier embodying one form of the invention; Figs. 3A, 3B and 3C are diagrams, and Fig. 3D a set of curves for facilitating explanation of the operation of the amplifier of Fig. 3; Fig. 3E shows a modification 0 of the amplifier of Fig. 3; Fig. 4 shows a feedback amplifier embodying another form of the invention; Fig. 4A is a simplified circuit diagram of the amplifier of Fig. 4, for facilitating explanation of the operation of that amplifier; Fig. 5 shows amodified form of the amplifier of Fig. 4; Fig. 6 shows a modified form of the amplifier of Fig. 5; and Figs. 7 and 8 show curves for facilitating explanation of the action `of a system of the type of that shown in Fig. 5.

The driving voltage in the plate circuit of an amplifier is out of phase with the grid voltage which produces it. For a feed-back amplifier, it is desirable that there be available a voltage which is directly proportional to, and in phase with, the driving voltage in the plate circuit, and which is 4independent of the impedance of the work circuit. It will be seen from Fig. l and the derivation below, that the voltage Ae which is the drop across resistances KR and KRO, fulfills these three conditions.

In the figure, RO is a resistance, (for example, the resistance of the space' discharge path between the plate and the filament of a three electrode voltage e produced in the discharge path by the The circuit of Fig. 2 is like that of Fig. l, except that a resistance R, is bridged across KRO and KR. The current components flowing in the various parts of the circuit of Fig. 2 are indicated by the arrows and their accompanying letters i f Fig. 3 shows one way in which this voltage may be utilized. In this figure a three-electrode, electric space discharge amplifying device I has an anode-cathode space-discharge path of resistance RO. which is associated with resistances KRO, KR, and R, and impedance Z, in the manner of resistanceR in Figs. 1 and 2. The platefilament resistance RO is the reciprocal of the slope of the static characteristic of plate current versus plate voltage of the discharge device at the so called operating point, as explained in the following articles by John R. Carson in the Profr ceedings of the 'Institute of Radio Engineers:

"Theory of three element vacuum tube, vol. 7. pp. 187-200, April 1919; The equivalent circuit of the vacuum tube modulator", yvol. 9, pp. 243-249, June 1921. v(Thai-fis, n

.R0- 58p where ip and ep are instantaneous plate current and voltage, respectively.) In Fig. 3 the impedance Z- is the primary-to-secondary impedance of'output transformer 2 which, together with circuit 3 connected to the secondary winding of the transformer, forms the load or work circuit for the device I. An input transformer 4 impresses waves from circuit 6 upon the grid of the device I. These waves may be, for example, voice waves, or voice modulated carrier waves for transmissionover carrier wave wire transmission systems or to radio transmitting antennae or waves received over such systems. 'I'he usual plate,.illament, and grid batteries are shown at 6, 1 and 8. A circuit corresponding to the resistance Re of Fig. 2 and comprising the secondary winding or secondary-to-primary impedance R01 battery 6 supplies plate current for the tube comprises resistance KRo in series with two parallel paths, one extending through the primary winding of transformer 2 and the other through resist-y ances KR and R in series.

A characteristic of this circuit of Fig. 3 is that the gain is reduced bythe feed-back action. To demonstrate this, it is simpler to redraw the circuit to the form of Fig. 3A. From this it will be seen that R and KR, Ra and KRO form the ratio arms of a balanced Wheatstone bridge, Z takes the place of the galvanometer and E represents a source of voltage E which is applied through a resistance of (-l-Roi) that forms the input diagonal or feed-back diagonal of the bridge. For the condition of balance, it is evident that there is no current in Z due to E, the driving voltage in the grid circuit.

There' is some voltage V, from grid to filament. Due to V, there will be a driving voltage aV in the plate circuit, with relative polarity as shown by the plus and minus signs. The presence of this voltage is indicated in Fig. 3A by generator. MV.

To investigate the effect of the two generators or voltages E and MV acting simultaneously, use is made of the principle of superposition. This principle is that the current which ows at any point in a circuit, or thefdiilerence of potential which is established between any two points in a circuit, due to the simultaneous action of a group of E. M. F.s located at the same or various points in a circuit, is the algebraic sum of. the component currents at the first point, or the component differences of potential between the latter two points, which would be established by the individual E. M. F.s acting alone.

Due to E alone, there will be a voltage from grid to filament, V1 equal to Due to aV acting alone, there will be a voltage from grid to filament V2 equal to or, if

These specific values for V1, V2 and l follow directly from the configuration of the specic circuit shown, but, though'special to that particular configuration, are given in order to present a concrete illustration of the significance of V1, V2 and ,61.

ylarity as shown. As before,

By the principle of superposition,

V=V1+V2 V=V1p1V l IF-(141031) That is, the voltage in the grid circuit (and hence the current in the output impedance Z) is reduced from what it would be if there were no feedback action, and the impedance relations were the same, by a factor l't-Mi It will now be shown that any distortion (as for example modulation product) produced in the tube is also reduced by the same factor.

Consider the circuit of Fig. 3B. Let any primary distortionV voltage produced in the tube be represented as a driving voltage ne, in the plate circuit. Let the voltage drop between the grid and thelament, resulting from the distortion produced i'n the tube be Vo. This sets up another driving voltage in the plate circuit, cVo, with po- That is, the final, or resultant, driving distortion voltage in the plate circuit is reduced by a factor f 1 1+#1 from what it would be without feedback action.

It will be noted that due to the configuration of the elements oi the circuit, if a generator is assumed in series with Z, no current will ow in the branch .'c-I-Roi. Hence the impedance of the amplifier as seen from the load or work circuit Z is the same as it would be without'regeneration, and is independent of the shunt resistance .r4-R01.

Now to return to a consideration of the gain of the circuit, by the feed-back action the gain is Astabilized with respect to variations of tubes and power. `If due to any cause, the a of the tube is reduced, which would reduce the gain, the

t effective voltage on the grid is increased. Similarly, variations in Rc are stabilized. The curves of Fig. 3D, which are plotted from observed data, show that by the feed-back action the load carrying capacity of the amplifier is substantially increased, the variation of gain with load is reduced, and the ratio of the power output of second harmonic to the power output of fundamental is improved or decreased about l2 decibels. The curves for operation without feedback are for operation with the right hand side of condenser 9 disconnected from the junction of R and KR and connectedv instead to ground through an impedance (not shown) equal to the impedance with which the feed-back diagonal of the Wheatstone bridge in Fig. 3 is faced by the remainder of the bridge (i. e., equal to the combined resistance of two paths in parallel, one through KR and KRO in series and the other through R and Ro in series). The resistance (a: -Ro1) ofthe feedusl , ponents.

4 would not affect the operation of the circuit. Thus, the curves for operation without feedback are for operation with the external grid-to-filament impedance and the external plate-to-filament impedance for the tube the same as in operation with feedback.

The circuit of Fig. 3 can be modified to comprise a plurality of tubes `connected in cascade, in which ease: a represents the total amplification from a voltage across the grid of the first tube to a. driving voltage in the plate of the last tube. For example, Fig. 3E shows one such modified circuit, with tandem connected tubes la, Ib and Ic replacing the tube l of Fig. 3. If the number of tubes be even instead of odd. then an odd number of phase reversals in addition to those produced by the tubes themselves should be produced, as for example by the introduction of an interstage or other transformer with its windings poled to reverse the phase -of waves passing through the transformer.

In Fig. 3A consider voltage Vs. Its value is being contributed to by both E and aV, and these voltages tend to oppose each other. It is possible, therefore, by suitable adjustment of the circuit elements, notably m, to make Va=0. The value of to accomplish this may be found as follows: I

In Fig. 3C.

' V= (Voltage drop across +V:

V== Voltage drop across n2) since V3=0 Similarly aV=iVoltage drop across (R4-Ro) l-i-Va =(Voltage drop across R-i-Rn) The current through a: must equal the current through (Rdf-R). for since Va==0, no current flows through (KR-i-KRo).

Y and a voltage is appliedto the circuit'in series with R01, no voltage is produced across the feedback diagonal of the bridge. (That is, Va in Fig. 3A is zero). This does not apply to distortion voltages, `since these are present in the plate circuit, but not in the grid circuit; i. e., only one generator is acting. By giving the value we have two points (the ends of the feed-back diagonal) across which the potential corresponding to the fundamental or original transmitted wave is zero, yand thepotential of the distortion or disturbing Wave is not zero. Across these points we have voltages corresponding to any disturbance present in the plate circuit which is not present in the grid circuit. We can now operate on `the distortion without disturbing the fundamental or originaltransmitted wave com-z For example, consider Fig. 4 and Fig. 4A. Fig. 4 shows a feed-back amplifier system comprising three vacuum tubes la, ib and Ic connected in tandem. Connected in seriesyacross the grid and filament of tube la, are a resistance i5 and the secondary winding of input transformer 4 which connects circuit 5 with the am' plier. The resistance I5 is sufllciently great to make the impedance of this resistance and the secondary-to-primary impedance of the transformer in series substantially a pure resistance RT. The tube ic is connected to the load or work circuit Z, as in the case of the tube I of Fig. 8 and the tube ic of Fig. 3E. As appears clearly from Fig. 4A, the impedance Zforms the outputl diagonal of the balanced Wheatstone bridge the ratio arms of which are R0, KRO, KR and R. In the feed-back diagonal of the bridge are a blocking condenser 9, a resistance RA and the resistance RT in series with each other and in `parallel with a circuit comprising a blocking condenser 9', resistance as, a blocking condenser i6, and the plate-to-lament space path resistance R01 of tube lb in series. Preferably, the admittance of the space current supply path, through battery 6b and choke coil I1, which path is connected across Rui, is negligibly small for the frequencies to be ampliiled. Bo also is the admittance of resistance i8 which is connected across the feed-back diagonal of the bridge. Biasing potential from battery 8c is applied to the grid of tube Ic through resistances I8 and :c in series. The blocking condensers4 9 and ily prevent plate battery 6c from applying potential to the grids of tubes la and Ic, Vand prevent grid biasing batteries 8a and 8c from applying potentials to the grids of tubes la and Ic, respectively. A re-l sistance I9 across circuit 5, and the primary-tosecondary impedance of transformer 4, in parallei, aiproximately match the impedance of that c rcui In the circuit'of Fig. 4 there will be no feedback of the fundamental or undistorted wave from the last tube lc to the first tube la, if :c has been given the proper value so that there is no fundamental or undistorted voltage across the feed-back diagonal of the bridge. It can be shown that the value obtained for a: in the above demonstrations holds when RA and RT and la and Ib are added to the circuit of Fig. 3C. However, the distortion introduced in the last stage will be reduced, as can be seen from the following consideration of Fig. 4A.

Let the distortion voltage acting in the plate circuit of the last tube be an@ after its reduction caused by feed-back in the last tube as explained in the case of the tube of Fig. 3 but before any reduction due to feed-back from the last tube to the first tube as about to be explained. There will be some distortion voltage Vo, impressed on the grid of the first tube as a result of the distortion produced in the last tube and Vo sets up a driving voltage /nVn that may be represented as a generator lnVo in the plate-to-filament space path of the second tube. This impresses a voltage tVo on the grid of the last tube, which sets up a driving distortion voltage #utVo that may be represented as a generator potVo in the plate-to-iilament space path of the last tube.

Since has been given such a value that the contribution of any voltage acting in series with Roi to V3 is zero, V0 is dependent solely on 09.

That is l RA-i-RT V .9 (1+ K" R0+R K (X-i-RniXRA'i-RT) X-i-'Roi-i-RA'i-RT or designating they bracket in this expression e2 Vo=uo29.

These specific values for Vo and p2 follow directly ali) ' shown, but, though special to that particular configuration, are given in order to present a concrete illustration of the significance of Vo and ,82. Then /LotVo=,u.oip.o29

Thaf; is, the nrw driving distortion voltage (no6-naive) is equal to the old driving distortion voltage #00, multiplied by a factor (1- /.io2t). Now if po2t=1, the distortion vanishes to zero. That is, if the product of the voltage amplifications and diminutions'around the circuit is l, and the phase shift is 180, the distortion voltage is balanced out. The amount of improvement in distortion in this circuit (over and above the improvement caused by feed-back in merely the last' tube) depends on the accuracy with which the circuit elements are adjusted in such a way that any distortion component fed back from the plate-filament space path of the last stage to the iirst stage, and amplified through the tubes again returns to the plate-iilament space path of the last stage with its original amplitude but with its phase shifted 186.

This operation of this circuit differs from that of Fig. 3 and Fig. 3E, in that the improvement in crosstalk in those circuits is obtained by continued regeneration of the distortion voltage, instead of by the balancing operation described above. In those circuit it is neither necessary nor probable that the voltage fed around the circuit will be equal to that originally present. The operation just mentioned in the circuit of Fig. 4 may be considered a single regeneration, with the voltage being fed around the circuit once, and coming back in opposite phase to that originally present.

The expressions continued regeneration and single regeneration as just used in reference to the feed-back amplifiers of Figs. 3, 3E, and 4, are in accordance with the usual explanation of the operation of feed-back ampliers, of which it is usually said, with reference to positive feed-back action respecting the fundamental frequency, that the operation wherein the energy of the output circuit and the input circuit react upon each other repeats'itself a number of times with cumulative eiiect. However, it is preferred to express the above noted difference in operation of va circuit such as that of Fig. 4 from circuits such as those of Fig. 3 and Fig. 3E by stating that in Fig. 4 the amplied, isolated distortion components fed to the input circuit of the tube lc do not cause the magnitude of the isolated distortion components (appearing across the input diagonal of the bridge) to be altered, whereas in Figs. 3 and 3E the distortion components fed back to the grid of the last tube cause reduction of the magnitude of the distortion components (that appear across the feed-back diagonal of the bridge).

Fig. 5 shows a three-stage feed-back ampliiier system similar to that of Fig. 4, but modified n in that instead of feeding the isolated distortion components (obtained across the feed-back diagonal of the Wheatstone bridge) back to the grid of the first stage or tube la and amplifying the distortion components and the signal components together in that tube before passing them on to the grid of tube Ib, those isolated distortion components are amplified separately from the signal, in an amplieryshown as comprising a single tube id (through which the signal components do not pass), and are then fed back to the grid of tube Ib. Thus, in the system of Fig. 5 the amplification of the isolated distortion components can be controlled :lndei,

pendently of the amplification of the signal com-v ponents, and can, for example, be made greater than the amplification oi' the signal components. The number of phase reversing amplifying stages (or other phase reversing means) in the path through which the distortion components are passed in their transmission from the feed-back diagonal of the bridge to the grid of tube ic should -be even. Thus, in this path, the number of phase reversing means or stages (such for example' as that comprising tube ib) in which the signal components and the distortion components are amplified alike should be odd or even according to whetheran odd or an even number of phase reversing means or stages (such as that comprising the tube id) are used in which the signal is not ampliiied as the distortion componentsare.

- In Fig. 5 the space current for tube lc is supplied rom battery 6c through a chokeL coil 20 of negligibly low admittance for the frequencies of the waves to be amplified, the current returning to battery 6c through resistance KRo.

`Condensers il, i2, i3 and i4 as Well as condensers e, s and i6, are stopping or blocking condensers which have negligibly low reactance at the frequencies to be amplified. If desired, the resistance KRO may be adjustable, as shown,

to facilitate balancing the Wheatstone bridge,l

the ratio arms of which are formed by the plateto-filament space path resistance Ro of tube Ic and the resistances'KRo, KR and R. The adjustment of KRq can be made to correct unbalance resulting from variations in plate impedance of tube lc caused for example by variations in the power supply voltages for the tube or by substitution of one tube for another. The impedance of the feed-back diagonal of the bridge can be adjusted by a Variable resistance 25 connected in parallel with a path comprising condenser 9', resistance :i: (shown adjustable), and plate-to-filament space path resistance Rc1 of tube ib in. series. Also in parallel with the re` sistance 25 is a path, of vnegligibly low admittance at the frequencies of the Waves to `be am# pliiied, extending through stopping condenser 9, input or coupling resistance RT for tube Id, and grid biasing battery 8d for that tube, in series. In parallel with Roi is a path through choke coil i1 and battery 6b in series, and also a path through grid biasing battery 8c and inputor coupling resistance i8"for tube lc. The two latter paths are of negligibly low admittance at the frequencies of the waves that are to be amplified. However, if desired, the magnitude of the combined resistance of these two paths and the space path of tube Ib may be used as the magnitude Roi in the formulae above. The resistance RT in Fig. 5 corresponds to the resistance RT in Figs. 4 and 4A and receives the voltage Vo indicated in Fig. 4A. In Fig. 5 there is no resistance corresponding to the resistance RA in Figs. 4 and 4A; or in other words the resistance RA is zero for Fig. 5. The battery 6b supplies space current for tube 1d through choke coil 26. Tube 'ld feeds back to the grid of tube lb through condensersI 3| and 32 and resistance 33 in series. This resistance adjusts the voltage thus yfed to that grid; and these condensers adjust the phaseof that voltage, the capacity of condenserV 32 being variable and rel- Figs. 4 and 4A. This the plate-to-filament space path of tube lb in.

Fig. 5, just `.asexplained above for the case of impresses the voltage tVg on the grid ofthe last tube le, which sets up l the driving distortion voltage autVo inthe plateizo-filament space path of tube Ic. As in the case of Fig. 4, the resistance :r has such a value vthat the contribution of any voltage acting in series with R01 to thev voltage V3 across the feedback diagonal of the bridge is zero, so that Vn is dependent `solely on ps9, the original distortion voltage in the plateo-filament space path of tube lc. i

In an amplifier system substantially as-shown in Fig. 5, if tube Ic in that figure be regarded as representing four tubes 'in parallel, the feed-back action reduced the energy contained in the second harmonic to 1/2,250,000 of its value without the feed-back action, and reduced the energy of the third harmonic 'to 1/50,200 of its value withf out the feed-back action. This resultentaiied no sacrifice of overall gain or ultimate level. Feed-back action of the type described for Fig. 3 tends to reduce the gain of the last stage (represented by tube Ic); but the circuit-of the type shown in Fig. 5 preferably is operated withthis tendency to gain reduction in the stage comprising tube lo not very pronounced, and with great reduction of distortionby the balancing operaf tion of the type described for Fig. 4. The gain plifiers.

.acteristia be apparent from the explanation given, for exof the first two stages, comprising tubes la and ib, canbe large, this portion of the amplifier serving as a voltage amplifier for stepping up the voltage to the power stage represented by tube Ic. The reason why the reduction of the second harmonic is greater than the reduction of the third harmonic (as appears from the 'ratios given above), `is primarily that although the second harmonic (and all even power harmonics) originated by distortion in the penultimate tube Ib, and also all even powerI sum and dinerence frequency components o-r waves so originated, are reduced by neutralizing or counteracting them i (as described 4for the circuit of Fig. 4)` with the corresponding distortion waves produced by distortion in the last stage comprising tube Ic, such balancing out does not occur for odd order harmonies or other distortion arising from the presence of odd power., terms in the amplifier char- The reasons for this difference will ample, in U. S. patents to H. S. Read, 1,464,111, August 7, 1923, and H. Nyquist 1,570,770, January 26, 1926, of the operation of tandem amplifiers in reducing distortion waves originating in the am- The preferable practice, in operation of the circuit of Fig. 5, is to adjust the value of :z: so that the amplitude of the fundamental in the work circuit is the same for.' operation of the system with feedback as for operation without feedback. The operation without feedback is obtained by disconnecting the right-hand side of condenser 3l from the junction of the choke coil 26 and the plate of tube id and connecting it instead to ground through aI passive impedance, not shown, equal to the resistance of the plate-to-iilament space discharge path in tube ld. To make the distortion aooaasa capacity.

'components balance out in the space path of the last amplifier as explained in connection with Fig. 4, that is to make Mo9=aotVo, the magnitude lof the fed back distortion components appearing across the feed-'back diagonal of the bridge is controlled by adjustingy resistance 25 in that diagonal, and the phase shift in tubes Id, Ib and ic and their associated circuits is compensated for by phase correcting means, asior example by oo ndenser 32 which can be adjusted to give the desired phase correction. A harmonic analyzer (not shown) can be connected across circuit 3 in order to tell when the best adjustment for 25 and 32 has been eiected. With the circuit of Fig. 5 operated in the manner just described, al-

though the third harmonic is not reduced below the value produced in the penultimate stage, the reduction of the third lharmonic originated by distortion in tube Ic is a very great improvement. Since the next to last stage (tube Ib) is operating at relativelyalo/w power level, and as a voltage amplifier working into `a high impedance, the modulation that it produces is very small, and

lhas generally been considered negligible.

Each of the systems shown in Figs. 3 and 4 is typical of a wide variety of possible circuits. The

principles of operation described for either or both of these types of circuits are applicable in general to devices capable of amplifying waves, for improvement of their operation. These principles are of very broad and general application.

Their application is by no means limited to operation intended to be mere amplification. The theory of these systemslhas been checked by careful tests of physical embodiments of the systems. Each of these systems reduces both odd order and even order distortion components at the same time, and can stabilize gain and afford high load One difference between Fig. 4 on the one hand and Figs. 3 and/3E on the other hand, lies in the relative seriousness of unavoidable phase shifts in the amplifier. Since the eectiveness of Fig. 4 depends on the production of two voltage waves of equal amplitude, and opposite sign, a good balance requires that the phase difference be very close to and that the gain of the circuit change very little after a balance is once attained. However, in Figs. 3 and 3E, the improvement depends on the factor 1 K 1+. indicating a factor such as l or -mentioned above.

If originally an improvement'of 20.8 db. If we assume the gain to change by 3 db., so that'a=7+i0,'then an improvement of 18.0 db. A similar change in gain in Fig. 4 would lchange an infinite improvement to a 10.4 db. improvement. Similarly, if s=7.07+j7.07 (absolute magnitude the same, but at a 45 angle) 1 PPMS-.1075

put of second harmonic, instead of 900,000,000

mented or made cumulative, by extending the circuit to form that of Fig. 6. Asecond bridge circuit is formed, with a ratio arm constituted by the network that faces the output diagonal of the first bridge. In this arm, therefore, is a voltage proportional to the residue of distortion remaining after the rst balancing. The fundamental or original transmitted wave components are also present in this arm. The other three ratio arms of the second bridge are KR'u, KR', and R', corresponding respectively to the ratio arms KRO, KR, and R of the rst bridge. The output or work circuit diagonal of the second bridge is the condenser H and the primary-to-secondary impedance of amplifier output transformer 2 in series. The resistance corresponds in function to resistance and is of such magnitude that the voltage of the fundamental or original transmitted wave components across Roi and .'r in series (i. e., across the feed-back diagonal of the second bridge) is zero. Thus the primary winding of a transformer 35, which is connected across the feed-back diagonal of the second bridge, receives only a voltage proportional to the residue of distortion just mentioned. This voltage is amplified in amplifier d and the amplied voltage is fed through resistance 33 and condenser 3l to the grid or input circuit of tube lb. It is apparent that the residue of distortion just mentioned is isolated from the fundamental components and fed back in the same fashion as the first process. Thus the distortion is again improved by approximately the same ratio as before. In balancing circuits which have heretofore been used for reducing distortion (such as the Colpitts push-pull circuit), only one improvement can be achieved, and that only for odd order or even order distortion components and not for both odd order and even order components at the same time; whereas -the improvement obtained with the circuit of Fig. 5 can be multiplied as many times as desired by adding bridges in tandem as indicated in Fig- 6, each additional bridge reducing both odd order and ev'en order distortion components and reducing the distortion by approximately the same factor as the circuit with only the rst bridge.

In Fig. 6 the elements 0", l2', i3', 25', 26', 3l', 33' and l 'd are similar in structure and function to the elements 9', l2, i3, 25, 25, 3l, 33 and ld, respectively, and the primary-to-secondary impedance RT of transformer 35 is connected across the feed-back diagonal of the second bridge as the impedance RT is connected across the feedback diagonal of the first bridge.

Fig. 7 is a set of curves plotted from observed data, showing the output of second and third harmonics as functions of output of the fundamental or original transmitted current', for a system of the type of that shown in Fig. 5. The curve for the second harmonic taken with regeneration shows that with an output of fundamental up to 30 milliamperes into an impedance of 600 ohms, the power level of the output of second harmonic is 90 decibels below the power level of the output of fundamental. This means that the fundamental power is about 900,000,000 times as great as the power of the second harmonic, and that the current ratio of fundamental 4)to second harmonic is about 30,000. The curve for the second harmonic taken without feedback shows that up to 30 milliamperes output of fundamental the output of second harmonic is only about 30 db. lower than the output of fundamental (i. e., the power output of fundamental is only about 900 times as great as the power outtimes as when feedback is employed). For current outputs greater than 30 milliamperes of fundamental, the curve for the second harmonic for operation with feedback is still well above the curve for the second harmonic ,for operation with# out feedback.l Likewise, the curves for the third harmonic taken with and without feedback show great reduction of the ratio of that harmonic to the fundamental, as a result of the feedback. Moreover, similar important reduction of the output levels of other harmonics. both odd and even, (and also of the output levels of intermodulation products), as compared to the output levels of the fundamental waves, is effected by the feedback. The curves for both harmonics for operation without feedback are for operation corresponding to operation of the circuit of Fig. 5 with the right hand side of condenser 3| disconnected from the yjunction of the choke coil 26 and the plate of tube ld and connected instead to ground vthrough a passive impedance (not shown) equal to the resistance of the plate-to-flament space discharge path in tube id.

Fig. 8 is a set of curves plotted from observed data, showing the overall gain as a function of current output into a 600 ohm resistance, for the fundamental, in a specific system of the type of that shown in Fig. 5. Below about 30 milliamperes of output current the solid li-ne curve, which is for operation with feedback, substantially coincides with the dotted line curve, which is for operation Without feedback. This is in marked contrast to the gain-load curves of Fig. 3D for the circuit of Fig. 3, inasmuch as the feedback in the latter circuit lowers the gain. In Fig. 8 the solid line curve lies well above the dotted line curve, for outputs considerably greater than thirty or forty milliamperes, thus showing great gain stabilization effected by the feedback, this stabilization being effected without gain reduction corresponding to such reduction indicated by-Fig. 3D for the circuit of Fig. 3.

It is desired to emphasize the fact that the invention increases the load carrying capacity of electric space discharge tubes (l) not only by attaining an increase in load capacity of very great importance by suppression of distortion components of frequencies other than the fundamental frequencies and thereby permitting the tubes to operate over a larger range of their grid-voltage plate-current characteristics but also (2) by attaining a seco-nd increase of very great importance in the load capacity by feedback of fundamental waves in such a way as to control gain in a desired manner, as for example, to prevent undesired lowering of gain, for the fundamental waves.

In connection with this latter increase, it should be noted that in each of the amplifying systems described herein the invention provides means for correcting for distortion caused by improper degree of amplification of fundamental waves, as for example caused by amplification of a fundamental wave of a given frequency different amounts for different input amplitudes, or as for example caused b v amplification of two Waves of respectively different fundamental frequencies, different amounts, respectively. If in Fig. 4 or 5, for example, a wave of a given fundamental 'frequency is amplified in tube Ic (or the circuits associated with the tube) to a degree less than, say, the normal amplification for the tube (and the associated circuits) then for that frequency the feed-back voltage, or regenerated voltage across the feed-back diagonal of the bridge tends to be lower than normal, i. e., less than the fundamental which is applied there from circuit through Roi and m. As a result, the tendency toward lower than normal gain of the system for the fundamental wave of the given frequency is checked. Similarly, if the given frequency is amplified to a degree greater, instead of less, than normal in tube Ic, then for that frequency the feed-back voltage tends to be higher than the .voltage of that frequency which is applied across the feed-back diagonal by circuit 5 through R01 and 3:; and as a result the tendency toward higher than normal gain of the system for the fundamental wave of the given frequency is checked. The system compensates for too low or too high gain for fundamental waves, at the same time that it suppresses components of frequencies other than fundamental frequencies.

For the sake of simplicity the invention has been explained above with reference especially to pure resistance impedances where impedances have been described as external or ratio arms -of Wheatstone bridges, or as input diagonals of the bridges. However, the invention is not limited to the case in which these impedances are resistances. They, as well as the load, may be impedances of any character, (proper provision being made, of course, for the necessary supply of steady potential to the plates and grids of the tubes). That is, generalized impedances Zo, KZo, KZz, Z, Zi, Zoi, ZA and ZT may replace Ro, KRL, KR, R, sc, R01, RA and RT, respectively. These replacements may likewise be made in the lmathematical formul and expressions above.

In any of the figures of the drawings the impedance Zn includes the tube internal plate-tofilament capacity. Where Ro has been treated as constituting one arm of the bridge, the platefllament capacity is so small that its reactance at frequencies of the order of those of the Waves to be amplified is so great compared to the impedance Ro as to be negligible.

What is claimed is: r

l. In combination, wave translating apparatus, means for deriving waves from waves produced in said apparatus, and means capable of transmitting waves of the frequency of said produced waves, for impressing the derived waves on the input side of said apparatus without thereby altering the intensity of the derived waves.

2. In combination, wave translating apparatus, means for supplying to said apparatus waves producing modulation in said apparatus, means for deriving waves from the modulation components, and means for impressing the derived waves on the input side of said apparatus without thereby causing alteration of the intensity of the derived waves.

3. A system comprising a wave translating device that'generates distortion components in response to application of waves to the input circuit of said device and that reverses the phase of waves transmitted through said device, means for deriving from said distortion components waves of frequencies exclusive of waves produced without frequency change by the applied waves, and amplifying means for amplifying the derived waves and impressing them on the input circuit of said device, in the same phase in which they are originally generated, Without altering the intensity of the derived waves.

4. A system comprising a plural odd number of stages of electric space discharge devices, means for balancing waves produced by the last one of said stages in response to waves of other frequencies against said waves of other frequencies, and means for feeding waves remaining after said balancing operation back to the first of said stages without thereby causing alteration of the magnitude of the latter waves. v

5. A wave translating system comprising two electric space discharge devices, means for supplying Waves from the output circuit of one of said devices to the input circuit of the other of said devices, and means for reducing harmonic waves generated in said other device in response to said waves to such a value that they are balanced out by harmonic waves of like order originated in said one device.

6. A system comprising a plurality of stages of electric space discharge devices, means for balancing waves produced by one of said stages in response to other waves against said other waves at the input of said stage, and means for feeding waves remaining after said balancing operation back to another of said stages preceding said one stage without thereby causing alteration of the magnitude of the latter waves.

7. The method which comprises so operating upon fundamental waves as to produce a resulting wave containing fundamental components of the frequencies of said fundamental waves and modulation products different from said funda- 'mental components, isolating said modulation products from said fundamental components by obtaining said fundamental waves at a point where they are undistorted and balancing said fundamental waves so obtained exclusive of other waves against said fundamental components inv said resulting wave, and transmitting said modulation products to said point and so regenerating said modulation products that they reappear at their place of origin with their phase reversed.

8. The method which comprises so operating upon fundamental waves as to produce a resulting wave containing fundamental components of the frequencies of said fundamental waves and modulation products different from said fundamental components, isolating said modulation products frorn said fundamental components by balancing said fundamental waves exclusive of other waves against said fundamental components in said resulting wave, and so regenerating said modulation products, without thereby altering the magnitude of the isolated modulation products, that the modulation products reappear at their place of origin with their original magnitude but in reversed phase.

9. Signaling apparatus comprising a wave translating device, means for transmitting fundamental waves to said device which produce therein a resulting wave containing fundamental components that have the same frequencies as said fundamental waves and distortion products differing from said fundamental components, means for deriving from a portion of said apparatus said fundamental waves exclusive of other waves and so opposing said derived waves to said fundamental components in said resulting wave as to isolate said distortion products, and means for feeding said distortion products back to said portion of said apparatus and so regenerating said distortion products in'said device that they reappear at their place of origin in reversed phase.

10. Signaling apparatus comprising an amplier, means for transmitting fundamental waves to said amplier which produce therein a resulting wave containing fundamental components that have the same frequencies as said fundamental waves and incidental distortion products differing from said fundamental components, means for so opposing said fundamental waves exclusive to other waves to said fundamental components in said resulting wave as to isolate said distortion products, and means for so regenerating saiddistortion products in said amplifier, without thereby altering the magnitude of the isolated distortion products, that the distortion products reappear at their place of origin with their original magnitude but with their phase reversed.

11. The method which comprises so operating upon fundamental waves as to produce a resulting wave containing fundamental components of the frequencies of said fundamental waves and distortion products different from said fundamentalcomponents, isolating said distortion products from said fundamental components by balancing said fundamental waves exclusive of other waves against said fundamental components in said resulting wave, amplifying said isolated distortion components in their isolated state, and thereafter so regenerating them, without thereby altering the magnitude of the isolated distortion components, that the distortion components reappear at their place of origin in their original magnitude but in reversed phase.

12. A circuit comprising a wave translating device, means for transmitting to said device fundamental waves which produce distortion components in said device, a wave transmission path for transmitting the fundamental waves from a point in said circuit anterior to said device substantially without distortion and opposing the waves so transmitted against waves transmitted through said device, said path having such transmission eiflciency and phase shift that the fundamental waves transmitted therethrough neu.

tralize the fundamental component of the opposed distorted waves from said device and thereby yield the distortion components without fundamental waves, and means for causing these isolated distortion components to be fed back to said point in said circuit and so regenerated that they reappear at their place of origin in reversed phase.

13. A circuit comprising a vacuum tube device. means for transmitting to said device fundamental waves which produce distortion components in said device, a path for transmitting the fundamental waves substantially without distortion and opposing the waves so transmitted transmitted therethrough neutralize the fundamental component of the opposed distorted waves from said device and thereby yield the distortion components without fundamental waves, and means for causing these isolated distortion components to be so regenerated in said device, without thereby altering the magnitude of the isolated distortion components, that the distortion components reappear at their place of origin with their original amplitude but .vith reversed phase.

14. The method of operating upon a. wave which comprises so amplifying the wave as to produce fundamental and distortion components, isolating the distortion components from the fundamentalcomponents, so regenerating the distortion components as to avoid altering the magnitude of the isolated distortion components but to balance the regenerated distortion components against the original distortion components to obtain their difference as relatively small resultant distortion components, deriving from said resultant components distortion components of reversed phase and substantially the same magnitude, and balancing said components of reversed phase against said resultant components.

15. In combination, wave translating apparatus, means for deriving waves from waves produced in said apparatus and including compoy 17. A wave translating system comprising two wave paths in parallel, vacuum tube apparatus in at least one of said paths, the respective numbers of vacuum tube stages in the two paths diifering by an odd number, and means for applying waves from the output of one of said paths to their input.

HAROLD S. BLACK. 

